Sea Level Rise

Figure 1. During the last ice age (top) sea level was at least 120 m lower than it is today (bottom), exposing much more area on the continents. In these topographical maps, you can see the many changes that took place as sea level rose, among them the disappearance of the land bridge from Siberia to Alaska, the appearance of Britain and the islands of Southeast Asia, and the filling of the Hudson Bay. Another good image can be found here. Image credit: National Geophysical Data Center (NGDC) at NOAA..

Measuring Sea Level

Well over seventy percent of our planet is covered by water. This was not always the case. During the last ice age, massive ice sheets covered portions of the northern and southern hemispheres (Figure 1). When the massive ice sheets melted, they released the water locked inside them – enough to make the seas rise by about 120m (390 ft) (Waelbroeck et al., 2002; Schneider von Deimling, et al., 2006; Rahmstorf, 2007).

Global mean sea level (MSL) has been rising since the end of the last ice age almost 18,000 years ago. Measures of sea level refer to the level of the ocean's surface halfway between high and low tide (to find out more about how sea level is measured, see our links below). Often, they are used to standardize measurements of land elevations and sea depths. Sea level, however, is not the same across the world. Mean sea level is the average, or mean, height of the sea.

For centuries, sea level was measured using tide gauges. Since the mid-20th century, and especially over the last decade, satellites have played an increasingly significant role in measuring sea level. In addition to monitoring current sea levels, scientists have been trying to understand the history of changes in the height of the oceans. Using archeological information gleaned from salt marshes and coral reefs, past sea levels can be established.

Sea Level Rise

Since the beginning of the 20th century, the seas have continued to rise at an average rate of 1.7 ± 0.5 mm per year, according to the IPCC (Bindoff et al., 2007). This increase, however, has not happened at a constant rate. The first noted increase was over the period of 1961 to 2003, when the average rate of sea level rise was 1.8 ± 0.5 mm per year (Church et al, 2001; Church and White, 2006; Bindoff et al., 2007). Global mean sea level measurements have become even larger since 1993. According to the IPCC, "For the period 1993 to 2003, the rate of sea level rise is estimated from observations with satellite altimetry as 3.1 ± 0.7 mm yr–1, significantly higher than the average rate."

The historical record from tide gauges shows that large rates have been measured during other periods since the 1950s other than from 1993 to 2003 (Bindoff et al., 2007). The global mean sea level has changed significantly throughout Earth's history. As mentioned earlier, during the last glacial maximum about 20,000 years ago when the average global temperature was 4° to 7°C colder, sea level was 120 m lower than it is currently (Waelbroeck et al., 2002; Schneider von Deimling, et al., 2006; Rahmstorf, 2007). In contrast, during the Pliocene three million years ago, the climate was 2° to 3°C warmer and the seas were 25–35 m higher than today (Dowsett et al., 1994; Rahmstorf, 2007).

In addition to experiencing variability over time, sea level is not the same everywhere (see Figure 2). Changes in sea level are also not uniform, as some areas find levels rising faster than others. Despite the world-wide trend of rising sea levels, in some places the MSL is currently falling.

Factors Driving Sea Level Rise

Sea level rise is due to a number of causes, some of which may exert a more regional influence than others. These include:

Thermal expansion – As seawater becomes warmer it expands. Heat in the upper layer of the ocean is released quickly into the atmosphere. However, heat absorbed by the deeper layers of the ocean will take much longer to be released and therefore, be stored in the ocean much longer and have significant impacts on future ocean warming.

An increase in freshwater inputs from mountain glaciers, ice sheets, ice caps, and sea ice, as well as other atmospheric and hydrologic cycles due to rising global surface and ocean temperatures

Physical forces – Subsidence and lifting are associated with tectonic activity and the extraction of water and resources such as gas and oil. These types of forces don't actually change the volume of the ocean, only the relative sea level. However, these changes do affect movement over land, as well as estimates from satellite altimetry. For example, in Scandinavia's Gulf of Bothnia, the weight of glaciers had caused the land beneath it to compress and sink. Now that glaciers are melting and the pressure has been released, the region is lifting at a rate of as much as 11 mm per year. This rebound makes it seem like sea level is dropping even though it is actually rising by 2.1 mm per year (Milne et al., 2001).

Ocean current variations – Large, regional ocean currents which move large quantities of water from one location to another also affect relative sea level without changing the actual volume of the ocean. For example, el Niño moves water from one side of the Pacific to the other every three or four years. These large-scale variations also affect the relative sea level of certain areas. In normal conditions, trade winds blow across the Pacific toward the west. According to NOAA, the trade winds push warm surface water to the west Pacific, so the sea level is roughly 1/2 meter higher in Indonesia than it is in Ecuador. During el Niño years, this warm water is pushed over to the eastern Pacific.

Atmospheric pressure influences sea level by impacting the surface itself. This also only affects relative sea level as the water pushed out of one place will move to another.

Contributions to Sea Level Rise Broken Down

Thermal expansion from current and future greenhouse gas sequestration and increasing mass losses from the world's glaciers and ice sheets due to warming oceans and surface temperatures represent the two primary ways that anthropogenic climate change and its associated feedbacks do and could continue to affect sea level. But what proportion of sea level rise is attributable to each? Also, where exactly is the ocean expanding the most? Where is all this melting ice coming from? The table in Figure 3 offers a quick summary of the sources of sea level rise and their related input.

Thermal expansion

Global mean ocean temperature has been rising because about 10% the heat energy produced by greenhouse gases during the past half-century has been trapped in the oceans.

Since the beginning of the century, the impact of thermal expansion on sea level rise has increased dramatically. During the previous half century, thermal expansion accounted for only about a quarter of the observed sea level rise. Yet during the last decade of that period, its impact on sea level increased to the point where it accounted for roughly half of all observed sea level rise. The IPCC states, "For the period 1961 to 2003, the average contribution of thermal expansion to sea level rise was 0.4 ± 0.1 mm yr–1, for the period 1993 to 2003, the contributions from thermal expansion (1.6 ± 0.5 mm yr–1)." In other words, the oceans have risen roughly 25mm since 1961 due to thermal expansion.

There is, of course, variability. The different layers of the ocean trap different amounts of heat, and therefore warm at various rates. Input from melting snow and ice also effects ocean warming.

Melting

Loss of mass from glaciers world-wide, as well as from the ice sheets of Greenland and Antarctica contributes another 1.2 ± 0.64 mm to sea level rise per year. This is practically all of the sea level rise not attributed to thermal expansion.

According to Meier et al. (2007), roughly 60% of ice loss contributing to sea level rise is from glaciers and ice caps and not from the two ice sheets of Greenland and Antarctica. Melt from these smaller glaciers has accelerated over the past decade, and may cause 0.1 to 0.25 m of additional sea-level rise by 2100. The authors of the 2007 study report, "At the very least, our projections indicate that future sea-level rise may be larger than anticipated and that the component due to [glaciers and ice caps] will continue to be substantial."

Figure 3. Sources of sea level rise and their contributions in mm per year for the periods 1961-2003 and 1993-2003 from the IPCC 2007 assessment. Image credit: modified from Bindoff et al., 2007.

Mass losses from the massive ice sheets covering Antarctica and Greenland also contribute significantly to sea level rise. Like mountain glaciers, scientists are finding accelerated rates of melt on the Antarctic and Greenland ice sheets. Shepherd and Wingham (2007) found "…much of the loss from Antarctica and Greenland is the result of the flow of ice to the ocean from ice streams and glaciers, which has accelerated over the past decade." Factors contributing to the accelerated mass losses in both Antarctica and Greenland could, in the words of Shepherd and Wingham (2007), rapidly counteract the snowfall gains predicted by present coupled climate models" over the course of the next century. If the ice sheets were to melt completely, global sea-level would rise by 70m (Rahmstorf, 2007).

Why doesn't it add up?

Together, thermal expansion and melting snow and ice contributed 2.8 ± 0.7 mm per year to sea level rise between 1993 and 2003 (Bindoff et al., 2007). However, that total was lower than the actual observed global mean sea level rise for period by about 10%. What is causing the rest of sea level rise? This discrepancy is often referred to as the sea level enigma, and scientists have offered a number of possible explanations.

There are still uncertainties related to our measurements of thermal expansion. Ocean temperatures, and therefore estimates of thermal expansion, are not available for the entirety of the ocean. Our ocean temperature measurements only extend 2000–3000m deep, and data has not been collected for all parts of the globe – specifically, the Southern Hemisphere, where significant inputs from Antarctica likely exist, has not been completely sampled. Much of the unexplained sea level rise could well be due to thermal expansion that has not been accounted for yet.

The contribution of glaciers, ice caps, and ice sheets may be understated. Scientists are still fine-tuning estimates of total volumes of mountain glaciers, ice caps, and ice sheets as well as of mass losses from all of these areas. Almost immediately following the release of the 2007 report, scientists found accelerating rates of mass loss for sea ice and in a number of Greenland, Antarctic, and mountain glaciers – rates much higher than predicted by the 2007 report. The latest IPCC assessment also was not able to incorporate the most recent data showing the years with largest increases in acceleration of ice loss.

In other words, the missing 0.3 ± 1.0 mm per year of sea level rise is likely due to incomplete data for both thermal expansion and mass losses for melting snow and ice. The current IPCC assessment reports a much smaller difference between observed rates of sea level and the estimated inputs to sea level rise from various sources than the previous IPCC report. Information gleaned from new areas, technologies, and studies allowed scientists to more precisely calculate rates and extent of thermal expansion as well as mass losses in glaciers, ice caps, and ice sheets. Other sources, such as terrestrial freshwater inputs, however, are still being considered and their roles in sea level rise continue to be studied.

Predictions for Future Sea Level Rise

Global sea level has been rising since the late 1700s, according to tide gauges measurements that began in Amsterdam in 1700, in Liverpool, England in 1768 and in Stockholm, Sweden in 1774. These gauges suggest that the rise has been accelerating at 0.01mm/yr^2, and if the conditions that led to this acceleration continue, we can expect sea level will rise by 1.1 ft (0.34 m) by 2100 (Jevrejeva et al., 2008). At a minimum, sea level rise during the 21st century should equal that of the 20th century, about seven inches (0.6 feet, 0.18 m). This is the lower bound given by the IPCC in its 2007 assessment, which projected sea level rise of 0.6 - 1.9 ft (0.18 - 0.59 m) by 2100. However, they cautioned in their report that due to the lack of knowledge about how melting glaciers behave, the actual sea level rise might be higher. Since the publication of the 2007 IPCC report, a number of scientists have argued that the IPCC's projections of sea level rise are too conservative.

Ice sheet dynamics are still poorly understood, and many key processes controlling ice flow in a warming climate are not adequately taken into consideration in the 2007 IPCC report. Additionally, the effects of basal lubrication, dynamic glacial thinning, and increased ice stream flow after the disintegration of buttressing ice shelves are not included in current ice sheet models. Feedback loops and their impacts on sea level rise may also be underestimated. Tweaking is still needed on models of thermal expansion (Church, 2007).

Additionally, the IPCC employed the same model to predict future sea level rise as was used to inaccurately calculate past increases – according to one scientist, "The models in the IPCC report underestimated the sea level rise that we have already observed by 40%" (Kinver). Another stated, "…very low sea-level rise values as reported in the IPCC TAR now appear rather implausible in the light of the observational data" (Rahmstorf, 2007).

New studies are emerging that take into account factors not included in the IPCC models, such as increased glacial flow. According to a study based on semi-empirical relationships between changes in sea level and global temperatures, we should experience on the order of 10 to 30 cm of sea level rise per °C increase in temperature (Rahmstorf, 2007). When this relationship is applied to the IPCC's 2007 scenarios for warming during the 21st century which range from 1.4° to 5.8°C, sea level projections are higher than the IPCC's published estimates. The model-based estimates for the IPCC range from .18 to .59 m (0.6 - 1.9 feet), whereas Rahmstorf (2007) projects 0.5 - 1.4 m (1.6 - 4.6 feet) of sea level rise. The most recent major paper on sea level rise, published by Pfeffer et al. (2008) in the journal Science, conclude that a "most likely" range of sea level rise by 2100 is 2.6 - 6.6 feet (0.8 - 2.0 meters). Their estimates came from a detailed analysis of the processes the IPCC said were understood too poorly to model--the ice flow dynamics of glaciers in Greenland and Antarctica. The authors caution that "substantial uncertainties" exist in their estimates, and that the cost of building higher levees to protect against sea level rise is not trivial.

By examining past rates of sea level rise between 200 A.D. and the present, Grinsted et al. concluded that ice sheet respond more quickly to temperature changes than the computer models used in the 2007 IPCC assessment are predicting. The authors estimate that "IPCC projections of sea level rise 2090 - 2099 are underestimated by roughly a factor of three". The authors predict that global sea level will be rising 11 mm/year by 2050, which is four times faster than the 20th century rise.

Finally, the scientist who is arguably the most visible and authoritative climate scientist in the world, Dr. James Hansen of NASA, stated (Hansen, 2007) "I find it almost inconceivable that business-as-usual climate change would not yield a sea level change of the order of meters on the century timescale" (IPCC business-as-usual (BAU) scenarios assume that emissions of CO2 and other greenhouse gases will continue to increase year after year). Hansen gives a hypothetical but potentially realistic scenario where the sea level rise due to ice sheet disintegration doubles every decade, leading to a 5 meter (16 foot) sea level increase over the next century. He notes that during the period 2 - 3 million years ago, CO2 levels were similar to today (350 - 450 ppm), and global temperatures were 2 - 3°C warmer, similar to what we expect by the end of the century. Yet, this Plio-Pleistocene world was "a dramatically different planet, without Arctic sea ice in the warm seasons and with a sea level 25 ± 10 m higher."

Impacts of Sea Level Rise

As the world's oceans rise, low-lying coastal areas will disappear (see Figure 4). Flooding of coastal areas will become more common and more severe as storm surges have easier access to these lower-lying areas. The occurrence of extreme high water events related to storm surges, high tides, surface waves, and flooding rivers will also increase. Flooding and loss of land will have significant impacts on humans, wildlife, and entire ecosystems.

Migratory marine organisms will most likely be able to adapt. However, the rate of sea level rise will hamper the successful migration of a number of organisms. As ocean levels rise, coastal and low-lying areas and ecosystems will be flooded. Higher sea levels will likely have significant impacts on the structure, function, and capacity of coastal and inland ecosystems, influencing their capabilities to perform ecosystem services.

Coastal development also creates obstacles to plant and animal life as they are trying to adapt to changes in the ecosystem. For example, in Bangladesh and Thailand, coastal wetlands and mangrove forests, which act as buffers to storm surges and tidal waves, are already being submerged by rising sea levels. The mangroves would normally re-establish themselves at the new low-tide zone, however, buildings and other types of development on the coast are blocking them.

These changes in coastal and terrestrial ecosystems and resources will consequentially impact ocean circulation as well as sediment and nutrient flow in coastal areas.

Impacts on Human Populations

More than half of the world's population currently lives in a coastal region. The United Nations Environment Program predicts that by 2010 about 80% of the world population will be living within 62 miles of the coast, and of those, 40 percent will live within 37 miles of the coast.

Coastal areas are also important economic areas as they are resource-rich. Tourism, aquaculture, fisheries, agriculture, forestry, recreation, and infrastructure will all be strongly affected by the effects of rising sea levels (Nicholls et al., 2007). For example, a 2001 study found that 90% of Guyana's population and "important economic activities are located within the coastal zone and are threatened by sea-level rise and climate change" (Khan, 2001).

Rising sea levels will lead to permanent and intermittent flooding in low-lying coastal areas across the world. Shorelines in Samoa have already retreated by as much as 160 feet, forcing residents to move to higher ground.

Although a 3–4 mm per year rise in global mean sea level sounds small, encroaching oceans have already consumed two small uninhabited islands of Kiribati in the Central Pacific. They are also threatening the existence of low-lying nations, especially islands in equatorial regions. Tuvalu's population is quickly trying to adapt to sea level rise of 5.7 mm per year. Homes across the island have been made uninhabitable by flooding and buildings and other coastal developments have been destroyed by hurricanes and swells. Tuvalu will likely be one of the first nations to disappear as sea level rise increases.

Wholesale and partial relocation of populations living in severely affected areas such as Tuvalu, Bangladesh, and Samoa has already created climate change refugees. Other areas where populations may need to be relocated include Bangladesh, the Maldives, Guyana, and the Netherlands. In areas where sea level is not projected to rise as fast, vast infrastructure projects have been put into place to lessen the impacts of rising sea levels such as in the Netherlands and London.

Residents of Tuvalu are also dealing with another effect of sea level rise – contamination of groundwater resources through salt water intrusion. Scientists are looking at the history of the Floridan aquifer system as an example of what may happen to coastal aquifers with continued sea level rise. There, the rapid 120-meter sea level rise at the end of the last glacial period flooded half of Florida, allowing salt water to intrude into the aquifer, where it continues to circulate today (Morrissey et al, 2008). Concern regarding water quantity in addition to water quality is rising. Increasing sea levels and flooding will have a number of impacts on terrestrial water storage in addition to seawater intrusion. These include rising water tables from salt water intrusion, erosion, and impeded drainage.

The potential public health impacts of continued and increased sea level rise are staggering. Already, coastal zones are experiencing contamination of groundwater resources and aquifers such as in Tuvalu. Flooding, both permanent and intermittent will allow diseases such as cholera and malaria to extend their ranges further inland. More frequent and intense severe storm events such as hurricanes and monsoons will also increase the number of cases and occurrences and duration of exposure to pathogens and diseases. The table below in Figure 5 offers more specific information about the public health impacts of sea level rise and its associated effects. However, vulnerability to public health impacts is also very much related to the preparedness and socio-economic status of individual nations, cities, towns, and villages.

Figure 5. The hazards associated with sea level rise and their public health impacts as outlined in the IPCC's 2007 assessment. Image credit: modified from Nicholls et al., 2007.